Bituminous paving machines, especially those for asphaltic paving of roadways and the like, consist of a tractor unit and a screed assembly. The latter is usually of the "floating" kind and is drawn by a pair of laterally spaced screed or pull arms pivoted to the sides of the tractor. The tractor itself is of the crawler-type or of the rubber-tired wheel-type, the drive sprockets of the former and the drive wheels of the latter being at the rear of the tractor. Both types are currently suspended in one of two fashions.
The first incorporates in effect a rigid rectangular body to whose four corners all the sprockets or all the wheels, as the case may be, are journaled in such manner that vertical travel of a track or a wheel owing to uneven terrain results in vertical movement of the tractor body. This is the least desirable of the two kinds of suspension because lifting of a forward corner of the body also lifts its corresponding rear corner and thus impairs traction. In the wheel-type lifting of a rear corner of the body also lifts the corresponding forward corner and impairs steering which is accomplished by the forward wheels. Since automatic grade and slope controls are mounted in effect upon the body, vertical movement of the latter impairs the quality of the mat laid by the machine. The best that can be said for this type of suspension is that it is relatively cheap and easy to design.
The other kind of suspension is the so-called "3-point" type and an example of this is shown in U.S. Pat. No. 3,453,939. From FIGS. 1, 6, 7 and 8 of that patent it will be seen that the stub axles 53 of the rear drive wheels 54 are fixed to the rear of the tractor body. Journaled about the axles 53, however, are the rear ends of a pair of side plates 60 which extend forwardly to the front of the tractor and on which the steered wheels 66 are journaled. The forward ends of the side plates 60 are journaled at 62 in turn to the ends of a "walking" cross-beam 63 which is centrally journaled at 64 to the tractor body. Hence the tractor body is suspended in effect at the three points 53 and 64 so that the front wheels 66 can move vertically without disturbing the rear drive wheels 54 and so affecting traction. Lifting of a rear wheel in turn does not affect steering, a more stable platform for automatic grade and slope controls is provided, and stress on the tractor body is reduced. In a crawler version the forward idler sprockets and the track bogies are all attached to the side plates 60. The "3-point" type of suspension is obviously superior to the other but it is more costly and more difficult to design properly.
Despite its manifest advantages the "3-point" type just described has some inherent disadvantages. For instance, and referring to FIGS. 2, 10 and 11 of the above patent, if the hopper 10 is unevenly loaded, say there is more material on the hopper dump 13 than on the other dump 13' (not an infrequent occurrence), weight will be lifted from the diagonally opposite rear drive wheel 54 and thus its traction impaired. The same result ensues in the case of the crawler version because weight will be lifted from the rear portion of the diagonally opposite track. Furthermore, the hopper dumps 13 and 13, when open must be at a sufficient elevation above the front wheels 66 (or the forward tracks in the crawler version) in order to accommodate vertical travel of the forward ends of the side plates 60, thus either decreasing the capacity of the overall hopper 10, in the crawler version, or, in the wheeled version, limiting the diameter of the wheels 66 which in turn diminishes their steering ability.
So the primary object of the present invention is an improved suspension of the "3-point" type for the tractor unit of a bituminous paver, whether of crawler or wheel-type, a suspension which not only eliminates the foregoing disadvantages of the conventional "3-point" type but also contributes several additional and significant advantages not, so far as is known, found in past or current bituminous pavers.